In industrial, medical, and various other applications, it often is necessary to make measurements at a resolution of less than 10 microns. This becomes especially difficult for samples, such as living samples, that are likely to move during the measurement(s). A particular problem arises when measuring dimensions of the eye, as the eye is highly likely to move between a pair of measurements taken on, for example, the front and back surfaces of the eye. One technology that has been used to measure eye-length is Optical Coherence Tomography (OCT), such as is described in U.S. Pat. No. 5,321,501, which is hereby incorporated herein by reference. OCT is a non-invasive, real-time imaging technique capable of obtaining images and making measurements on the order of about 10 microns or less.
Several variations of OCT systems have been used with varying results. One such variation is a time-domain OCT (TD-OCT) system. A TD-OCT system typically uses a standard Michelson interferometer-based approach, such as is described in U.S. Pat. Nos. 5,329,321 and 5,387,951 for a full reference path scan. In applications such as measuring eye lengths, it is desirable to form high-resolution axial scans of the front and rear portions of the eye, but there is little value to scanning the center of the eye. A long axial scan covering the entire length of the eye, with the desired axial resolution throughout the scan, would spend considerable time scanning through the center of the eye, during which the eye may move and during which time little or no valuable information is being collected. Therefore a system is desired that is capable of separate high-resolution scans of the front and rear portions of the eye. U.S. Pat. Nos. 6,053,613 and 6,775,007 and Publication No. 2005/0140981 describe OCT systems with dual reference paths and a short-range reference-path scan. All of these references are hereby incorporated herein by reference. FIG. 1 shows a basic full range reference path length scan system, wherein light from a low coherence source 102 is input into a beam splitter 104. The beam splitter directs the light along two arms, namely a measurement arm 106 and a reference arm 108. An optical fiber 110 in the measurement arm 108 extends into a device 112 that scans an eye 114 with a beam of light. The reference arm 106 provides a variable optical delay using light reflected back by a reference mirror 116. A piezoelectric modulator based path length stretcher 118 can be included in the reference arm 106 with a fixed reference mirror 116 to either change the path length or to create a modulation or beating frequency, and the reference mirror 116 can be a scanning mirror such that the mirror can be scanned in the direction of the incoming beam, shown in the figure as the Z-direction. The reflected reference beam from the reference arm 106 and the scattered measurement beam from the sample arm 108 pass back and combine through the splitter 104 to a detector device 120, including processing electronics, which can process the signals using techniques known in the art. The processed signals can be used to give a measurement of eye length, as well as to produce a backscatter profile or image on a display unit 122. Due to the need for the reference mirror to move, measurements may not be accurate since the position of the eye can change while the mirror is moving.
Another variation is a spectral domain OCT (SD-OCT) device, such as is described in U.S. Pat. No. 5,975,699, which is hereby incorporated herein by reference. An exemplary SD-OCT device uses a spectral interferometer, where light from a source such as a superluminescent diode (SLD) can be reflected from surfaces of the eye, the eye reflected light is combined with a reference light and is focused onto a spectrometer, where the resulting spectrum can be analyzed as known in the art. A problem with such an approach is that the depth range that can be covered in such a system is typically limited (by the spectral resolution of the spectrometer) to a few millimeters, such as 3-4 mm for a typical A-scan, which is not sufficient to cover the full length of the eye (an optical path length typically equivalent to 28 mm to 35 mm in air). Thus, measuring the eye length with a typical SD-OCT system requires two separate measurements between which the reference arm is moved by approximately the length of the eye. Since the eye can move during those two measurements, error can be introduced. A similar problem is encountered when using a swept-source OCT (SS-OCT) device that varies the wavelength of the light source, such as is described in U.S. Pat. Nos. 5,347,327 and 5,347,328, which are hereby incorporated herein by reference.
There are many other variations of these OCT devices, each of which suffers the same problem of eye movement during the measurement process. When using an interferometer to measure the relative positions in space of the “front” and “back” surface of the eye, here the front surfaces of the cornea and retina, respectively, it can take on the order of about ¾ second between the front and back measurements. This leads to the probability of movement. For high-precision applications such as cataract surgery, where a lens of the appropriate focal length is inserted in the eye, an error in measurement due to movement of just 100 microns produces an intolerable error of approximately 0.25 diopters in lens prescription, so it is critical to know the correct eye length (as used in industry, the distance from the front surface of the cornea to the front surface of the retina). Existing OCT systems therefore are not sufficient to measure this critical dimension.